1. Field of the Invention
The invention relates to frequency multipliers and more particularly to frequency multipliers of very low phase noise.
2. Prior Art
In electronic systems, in which very stable frequencies are sought, the frequency of the primary oscillator is controlled by a mechanically resonant crystal. The resonant frequency of the crystal, and in turn that of the oscillator, determined by the physical boundaries of the crystal, the larger crystals resonating at lower frequencies and the smaller crystals resonating at higher frequencies. The resonant frequency is further dependent upon the bulk properties (elasticity and density) of the crystal, which in turn set the velocity at which elastic waves constituting mechanical resonance propagate between the boundaries of the crystal. A further property of the crystal, attributable in part to the regularity of its internal molecular structure, is that the bulk elastic wave motion is accompanied by very low dissipative losses. Mechanical "Q"s as high as tens of thousands may be achieved with a crystal. The advantage of a higher "Q", is greater precision in defining the resonant frequency of an oscillator.
The crystal, i.e. a piezo-electric crystal, is, without serious competition as the frequency reference of choice in electronic systems when accuracy is paramount. Purely electrical resonators using inductors and capacitors or distributed structures such as wave guides and cavities rarely if ever attain Q's comparable to those of crystals.
However, the piezo-electric crystal cannot be operated at microwave or millimeter wave portions of the radio spectrum. The dimensions of the crystal, which must be reduced as the frequency is increased, cannot be reduced indefinitely. With the use of overtones (i.e. the fifth), crystals may be satisfactorily operated to about 100 MHZ--but operation at one gigahertz is not currently practical. One hundred MHZ is a factor of 10 below 1 gigahertz, and a factor of 100 below 10 gigahertz, the latter having operating frequencies of current interest in electronic systems.
Thus, when precise frequencies in the microwave or millimeter frequency spectrum are sought, the choice is a crystal controlled oscillator supplemented by electronic frequency multiplication. Ideally, frequency multiplication retains the precision of the original crystal in setting the final frequency. Conventional electronic multiplier circuits readily synthesize the second or third harmonic. Thus to achieve frequency multiplications of higher numbers than 2's or 3's, several frequency multiplier stages connected in cascade are employed. In order to achieve operation at 1 GHZ, for instance, using a crystal controlled oscillator, one must use multiple stages of frequency doubling or tripling.
However, known frequency multiplier stages are not ideal and are subject to short duration disturbances which create amplitude and phase noise which creates a new problem in critical applications. The conventional doubler and conventional tripler utilize active gain elements which are driven into saturation to give rise to the second or third harmonic. The noise performance of conventional multipliers are classically described and predictable. A decrease in the single sideband phase noise performance of 6 DB per stage is predicted for a doubler stage and approximately 18 DB for three stages of doubling. For a tripler, using the same mode of prediction, the decrease the single sideband phase noise ratio is approximately 9.54 DB per stage. Experience is generally worse than prediction, and the predicted deterioration in the signal to noise ratio, assuming conventional harmonic generation, is ordinarily taken as the ideal, which cannot be bettered.
One such critical application is moving target indication radar systems. Here phase noise may mask slowly moving targets. Slowly moving targets create very small changes in phase and frequency of the radar return, and these changes lie in a portion of the spectrum clustering around the carrier within a few hundred or a few thousand cycles per second. The phase noise contributed by subsequent frequency multiplier stages also clusters around the carrier and tends to mask the returns from the slowly moving targets. Thus any lowering of the phase noise in the eventual microwave or millimeter wave carrier will result in an improvement of the ability of the MTI system to detect more slowly moving targets.